1. Technical Field
This invention relates generally to wireless local area networks, and more particularly, to wireless local area networks employing physical layer modulation and demodulation in accordance with the IEEE Standard 802.11b—1999 Supplement (“IEEE802.11b”) to the ANSI/IEEE Standard 802.11, 1999 edition.
2. Related Art
There are several known techniques for transmitting digital waveforms across wireless networks. One known technique is direct sequence spread spectrum (DSSS), which allows for high-rate modulation using complementary codes known as “spreading codes.” The use of spreading codes enables the bandwidth occupied by a DSSS waveform to be increased or “spread.” As a consequence of this bandwidth spreading (and despreading), DSSS systems are able to realize processing gains compared to systems using other transmission techniques.
Complementary Code Keying (CCK) is the modulation technique chosen for IEEE 802.11b high rate modes (5.5 Mbps mode and 11 Mbps mode). For example, a CCK modulated symbol c may be expressed as:c={ej(φ1+φ2+φ3+φ4),ej(φ2+φ3+φ4),ej(φ1+φ2+φ4), −ej(φ1+φ4),ej(φ1+φ3),−ej(φ1+φ2),ejφ1}  (1)where φ1, φ2, φ3 and φ4 are suitable phase values as described in more detail below.
For clarity of description, the chips in equation (1) are hereinafter referenced from left to right as c0–c7, respectively. In CCK modulation, the 4th and 7th chips, namely c3 and c6, are rotated 180° to optimize the correlation properties and reduce DC offset.
When operating in the 5.5 Mbps CCK mode (4 bits/symbol), the various phase values φ1, φ2, φ3 and φ4 employed in equation (1) are defined as shown below in equation (2).
                    {                                                                              φ                  1                                =                                  DQPSK                  ⁢                                                                          ⁢                  encode                  ⁢                                                                          ⁢                  with                  ⁢                                                                          ⁢                                      (                                          d0                      ,                      d1                                        )                                    ⁢                                                                          ⁢                  and                  ⁢                                                                          ⁢                  even                  ⁢                                      /                                    ⁢                  odd                                                                                                                          φ                  2                                =                                                      (                                                                  d2                        *                        2                                            +                      1                                        )                                    *                                      π                    /                    2                                                                                                                                            φ                  3                                =                0                                                                                                          φ                  4                                =                                  d3                  *                  2                  *                                      π                    /                    2                                                                                                          (        2        )            where d0, d1, d2 and d3 are the 4 bits to be modulated.
When operating in the 11 Mbps CCK mode (8 bits/symbol), the various phase values are defined as shown below in equation (3).
                    {                                                                                                             φ                    1                                    =                                      DQPSK                    ⁢                                                                                  ⁢                                          encode  with                                        ⁢                                                                                  ⁢                                          (                                              d0                        ,                        d1                                            )                                        ⁢                                                                                  ⁢                                          and                                        ⁢                                                                                  ⁢                                          even                                        ⁢                                          /                                        ⁢                                          odd                                                                                                                                                                φ                    2                                    =                                                            (                                                                        d2                          *                          2                                                +                        d3                                            )                                        *                                          π                      /                      2                                                                                                                                                                φ                    3                                    =                                                            (                                                                        d4                          *                          2                                                +                        d5                                            )                                        *                                          π                      /                      2                                                                                                                                                                φ                    4                                    =                                                            (                                                                        d6                          *                          2                                                +                        d7                                            )                                        *                                          π                      /                      2                                                                                                                              (        3        )            where d0, d1, . . . , d6 and d7 are the 8 bits to be modulated.
When demodulating, the d2–d3 bits (5.5 Mbps mode) or the d2–d7 bits (11 Mbps mode) will be decoded by the CCK correlator, and d0–d1 by DQPSK demodulation.
The published CCK 64-vector correlation can be written as:
  R  =                              C          T                ⁡                  [                                                                      ⅇ                                      j                    ⁡                                          (                                                                        φ                          2                                                +                                                  φ                          3                                                +                                                  φ                          4                                                                    )                                                                                                                                            ⅇ                                      j                    ⁡                                          (                                                                        φ                          3                                                +                                                  φ                          4                                                                    )                                                                                                                                            ⅇ                                      j                    ⁡                                          (                                                                        φ                          2                                                +                                                  φ                          4                                                                    )                                                                                                                                            ⅇ                                      j                    ⁢                                                                                  ⁢                                          φ                      4                                                                                                                                            ⅇ                                      j                    ⁡                                          (                                                                        φ                          2                                                +                                                  φ                          3                                                                    )                                                                                                                                            ⅇ                                      j                    ⁢                                                                                  ⁢                                          φ                      3                                                                                                                                            ⅇ                                      jφ                    2                                                                                                      1                                              ]                    *        =                                                                                        C                  T                                ⁡                                  [                                                                                                              ⅇ                                                      jφ                            2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              ⅇ                                                      jφ                            2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              ⅇ                                                      jφ                            2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              ⅇ                                                      jφ                            2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              ]                                            *                        ⁡                          [                                                                                          ⅇ                                              jφ                        3                                                                                                                                                                                                                                    1                                                                                                                                                                                                                                                                                                                  ⅇ                                              jφ                        3                                                                                                                                                                                                                                    1                                                              ]                                *                ⁡                  [                                                                      ⅇ                                      jφ                    4                                                                                                      1                                              ]                    *      where CT=(c0, c1, c2, −c3, c4, c5, −c6, c7) (In-phase and Quadrature signal).
FIG. 1 depicts a CCK correlator architecture of the prior art. Only one phase or vector is shown for each of the φ values. It should be appreciated that the CCK correlator architecture depicted in FIG. 1 is capable of operating at either of 5.5 Mbps mode or 11 Mbps mode. As such, the actual hardware implementation and the time cost for both 5.5 Mbps and 11 Mbps modulation are the same, and therefore the power consumption is the same. In 5.5 Mbps modulation mode, φ3 is always equal to zero (see equation (2) above). Because the amount of real vector used for 5.5 Mbps modulation is less than the amount used for 11 Mbps modulation, the prior correlator wastes substantial power when operating at 5.5 Mbps and consumes as much power as is required for 11 Mbps operation.
Further, it has been proposed to further enhance CCK symbol modulation processing gain through decision feedback analysis based on e.g. previous symbol information and/or predicted subsymbol regeneration. Therefore, it would be desirable to implement power saving correlation techniques which could conveniently include decision-directed equalization using selective subsymbol prediction and regeneration for improving overall symbol correlation and demodulation.